Emissionless Oxyfuel Combustion Process and a Combustion System Using Such a Process

ABSTRACT

A method of combusting carbonaceous fuel in a combustion system. The combustion system includes a source of oxygen and a furnace. The method includes the steps of (a) feeding fuel and combustion gas including oxygen and recycling gas into the furnace for combusting the fuel with the oxygen and producing exhaust gas that includes CO 2 , water and excess oxygen as its main components, (b) conducting the exhaust gas discharged from the furnace into a scrubber so as to remove pollutants from the exhaust gas, (c) dividing the exhaust gas into a first exhaust gas stream and a second exhaust gas stream, and conducting the second exhaust gas stream as a recycling gas stream into the furnace, (d) conducting the first exhaust gas stream into a CO 2  purification and capturing unit (CPU) to produce one or more condensate streams, a purified liquid CO 2  stream and a vent gas stream that includes remaining CO 2 , (e) discharging the purified liquid CO 2  stream from the combustion system, (f) conducting the vent gas stream into an adsorption unit so as to adsorb compounds, including remaining CO 2 , from the vent gas stream to an adsorbing material and to produce a pass-through gas stream, and (g) regenerating the adsorbing material to produce a desorbed gas stream that includes at least a portion of the adsorbed compounds, and conducting at least a portion of the desorbed gas stream into the furnace.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an emissionless oxyfuel combustion process and a combustion system using such a process. More particularly, the present invention relates to an oxyfuel combustion process and combustion system with an emission recycling loop.

2. Description of the Related Art

The exhaust gases of a conventional power station typically contain from about 4% (by volume) to about 14% carbon dioxide (CO₂). It is commonly believed that this CO₂ represents a significant factor in increasing the greenhouse effect and global warming. Therefore, there is a clear need for efficient methods of capturing CO₂ from exhaust gases so as to produce a concentrated stream of pressurized CO₂ that can readily be trans-ported to a safe storage site or to a further application. CO₂ has been captured from gas streams by means of four main technologies: absorption, in which CO₂ is selectively absorbed into liquid solvents; membranes, in which CO₂ is separated by semipermeable plastics or ceramic membranes; adsorption, in which CO₂ is separated by adsorption on the surfaces of specially designed solid particles; and, low temperature/high pressure processes, in which the separation is achieved by condensing the CO₂.

At present, the most proven technique to capture CO₂ from an exhaust gas is to scrub the exhaust gas with an amine solution to absorb CO₂ to the solution. This technology has reached the commercial state of operation for CO₂ capture systems from small scale exhaust gases. However, application thereof considerably decreases the total efficiency of the combustion system. Another difficulty is that, in order to minimize contamination of the solvent by impurities, effective measures are necessary for cleaning the exhaust gas, for example, from sulphur and nitrogen oxides.

Oxyfuel combustion systems use oxygen, usually produced in an air separation unit (ASU), instead of air, for the combustion of the primary fuel. The oxygen is often mixed with an inert gas, such as recirculated exhaust gas, in order to keep the combustion temperature at a suitable level. Oxyfuel combustion processes produce exhaust gas having CO₂, water and excess oxygen (O₂) as its main constituents, the CO₂ concentration being typically greater than about 70% by volume. Therefore, CO₂ capture from the exhaust gas of an oxyfuel combustion process can relatively simply be done by using refrigerated separation at an elevated pressure. The water vapor is usually removed from the exhaust gas of an oxyfuel combustion process by cooling the exhaust gas before, during and after the compressing of the gas. Further treatment of the exhaust gas may be needed to remove air pollutants and non-condensed gases (such as excess oxygen) from the exhaust gas before the CO₂ is separated to be sent to storage.

U.S. Pat. No. 6,898,936 discloses an oxyfuel combustion process with exhaust gas recirculation, in which a portion of the exhaust gas is cooled and compressed in several steps to condense liquids with acid gases dissolved therein, or to directly condense acid gases, such as CO₂, from the exhaust gas. The resulting uncondensed gas stream, which is O₂ rich, may be conducted to an air separation unit (ASU) as a high-quality feed stream. A drawback in this process is that even if relatively high pressures and low temperatures are used, the uncondensed gas stream may comprise a considerable amount of CO₂, which may then be released to the atmosphere together with the vent gases of the ASU.

U.S. Pat. No. 6,574,962 discloses an oxyfuel combustion process with exhaust gas recirculation, in which a portion of the exhaust gas is cooled in several condensers to condense water, nitrogen oxides (NO_(x)) and sulphur oxides (SO_(x)) from the exhaust gas, and further cooled and compressed so as to condense CO₂ from the exhaust gas. The resulting uncondensed gas stream, rich in O₂, is combined with a cold O₂ rich gas stream from an ASU, which combined gas stream is then used as a cooling stream in the condensers and conducted as combustion gas to the furnace. A drawback in this process is that the uncondensed gas stream may comprise a considerable amount of N₂ and Ar, which will be recycled to the furnace.

U.S. Patent Publication 2008/0184880A discloses an oxyfuel combustion process wherein a vent gas stream from a primary CO₂ separating unit, producing a primary liquid CO₂ stream, is conducted to a temperature swing adsorption (TSA) based CO₂ separating unit. A CO₂ stream released from the TSA unit is combined with the primary liquid CO₂ stream, and a pass through stream of the TSA unit is conducted as a feed stream to an ASU. A drawback of this process is that the CO₂ stream released from the TSA unit may comprise a considerable amount of impurities, such as N₂ and O₂, which will then remain as impurities in the combined liquid CO₂ stream.

As becomes clear from the above-described problems and drawbacks of the prior art solutions, there is still a need for a simple and environmentally efficient oxyfuel combustion process.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an efficient method and combustion system for emissionless oxyfuel combustion.

According to one aspect, the present invention provides a method of combusting carbonaceous fuel in a combustion system, which combustion system includes a source of oxygen and a furnace, the method comprising the steps of: (a) feeding fuel and combustion gas comprising oxygen and recycling gas into the furnace for combusting the fuel with the oxygen and producing exhaust gas comprising CO₂, water and excess oxygen as its main components; (b) conducting the exhaust gas discharged from the furnace into a scrubber so as to remove pollutants from the exhaust gas; (c) dividing the exhaust gas into a first exhaust gas stream and a second exhaust gas stream and conducting the second exhaust gas stream as a recycling gas stream into the furnace; (d) conducting the first exhaust gas stream into a CO₂ purification and capturing unit (CPU) so as to produce one or more condensate streams, a purified liquid CO₂ stream and a vent gas stream comprising remaining CO₂; (e) discharging the purified liquid CO₂ stream from the combustion system; (f) conducting the vent gas stream into an adsorption unit so as to adsorb compounds, including remaining CO₂, from the vent gas stream to an adsorbing material and to produce a pass through gas stream; and (g) regenerating the adsorbing material to produce a desorbed gas stream, comprising at least a portion of the adsorbed compounds, and conducting at least a portion of the desorbed gas stream into the furnace.

According to another aspect, the present invention provides a combustion system for combusting carbonaceous fuel, comprising: a source of oxygen; a furnace; means for feeding fuel and a combustion gas comprising oxygen and recycling gas into the furnace for combusting the fuel with the oxygen and producing exhaust gas comprising CO₂, water and excess oxygen as its main components; an exhaust gas channel connected to the furnace for discharging the exhaust gas from the furnace; a scrubber arranged in the exhaust gas channel for removing pollutants from the exhaust gas; means for dividing the exhaust gas into a first exhaust gas stream and a second exhaust gas stream; a recycling channel for conducting the second exhaust gas stream as a recycling gas stream into the furnace; an end portion of the exhaust gas channel for conducting the first exhaust gas stream into a CO₂ purification and capturing unit (CPU) to produce one or more condensate streams, a purified liquid CO₂ stream and a vent gas stream comprising remaining CO₂; means for discharging the purified liquid CO₂ stream from the combustion system; an adsorption unit arranged to adsorb compounds, including remaining CO₂, from the vent gas stream to an adsorbing material and to produce a pass through gas stream; means for regenerating the adsorbing material to produce a desorbed gas stream, comprising at least a portion of the adsorbed compounds; and a desorbed gas channel for conducting at least a portion of the desorbed gas stream into the furnace.

Typically about 90-95% of the CO₂ in the exhaust gas is captured by the CPU. Thus, the vent gas stream discharged from the CPU comprises mainly O₂, N₂ and CO₂, and also some Ar (argon). The principal goal of the adsorption unit is to capture as much as possible of the CO₂ remaining in the vent gas. According to a preferred embodiment of the present invention, the conditions in the CPU are adjusted so that the vent gas stream is close to CO₂ dew point, typically at a temperature from about −50° C. to about 20° C. and at a pressure from about 20 bar to about 40 bar. Thus, the vent gas is suitable for very efficient CO₂ separation in a low temperature adsorption process. By suitable selection of the adsorption material and operating conditions of the adsorption unit, almost all, preferably at least 90%, even more preferably at least 95%, of the CO₂ in the vent gas stream is adsorbed in the adsorption material, and the pass through gas is nearly CO₂ free. The total CO₂ capturing efficiency is thus advantageously at least 99.5%.

The source of oxygen is advantageously an air separation unit (ASU), preferably a cryogenic ASU. According to a preferred embodiment of the present invention, at least a portion of the pass through gas stream of the adsorption unit is expanded to a suitable pressure and conducted as an additional low temperature feed gas stream to the ASU. This is advantageous especially in case the O₂ content of the pass through gas is higher than that of air. The present invention differs from the process disclosed in U.S. Pat. No. 6,898,936 in that, due to the efficiently CO₂ adsorbing adsorption unit downstream of the CPU, the pass through gas stream does not comprise any significant amount of CO₂. Thus, when the pass through gas stream is conducted to the ASU, it does not provide any significant amount of CO₂, which would be released to the atmosphere together with the vent gas, mainly N₂, of the ASU.

According to a preferred embodiment of the present invention, the adsorption unit is a pressure swing adsorption (PSA) unit. The adsorbing material bed of the adsorption unit may advantageously comprise activated carbon, zeolite or other suitable adsorbing material. When the adsorption material is about to be saturated, the material is regenerated, in case of a PSA process, by reducing the pressure to a suitable low pressure, typically about 1 bar, and a gas stream is desorbed from the material and discharged from the adsorption unit. The adsorption unit usually comprises in practice at least two adsorption beds, so that at least one bed can always be operational, even when regeneration of another bed is being made.

According to a preferred embodiment of the present invention, the gas stream desorbed from the adsorbing material is completely recycled back to the furnace. Then, assuming that all CO₂ of the vent gas stream of the CPU is adsorbed in the adsorption unit, not any CO₂ is released to the atmosphere. Then all CO₂ produced in the combustion system is captured by the CPU. In practice this is achieved so that the CO₂ content of the exhaust gas raises to a level, where the amount of CO₂ captured by the CPU corresponds to the amount of CO₂ produced by the combustion process.

An advantage of the present invention is that it is not necessary to aim for a sharp cut of the adsorbed molecule species at the adsorption unit, but a portion of the N₂ and O₂ in the vent gas stream may also be adsorbed in the adsorbing material. When practically all CO₂ and some of the N₂ and O₂ of the vent gas stream are adsorbed in the adsorption unit, practically all pollutants which may remain in the vent gas stream, especially NO_(x) and CO molecules, are typically also adsorbed in the adsorbing material. Thus, when the desorbed gas stream is conducted back to the furnace, also some N₂ and O₂ and all remaining pollutants, especially NO_(x) and CO molecules, are recycled back to the furnace.

As described above, the combustion system forms two closed loops: direct recycling of the second portion of the exhaust gas and the recycling of the compounds desorbed from the adsorption unit, without releasing any emissions to the atmosphere. As the pollutant compounds are recycled back to the furnace, a portion of the compounds, especially CO, SO₃ and NO_(x) molecules, as well as possible VOC-compounds (volatile organic compounds), are destructed in the furnace. The destructing of the pollutant compounds is important because otherwise the pollutant levels in the exhaust gas might increase to relatively high values before a balance is reached between the forming and capturing of the pollutants.

Due to the pollutant recycling, conventional NO_(x) reducing processes of the combustion system function very effectively even without high NO_(x) removal efficiency during a single cycle. Therefore, it is possible to use relatively low NH₃/NO_(x) ratio in a conventional NO_(x) removing process. According to a preferred embodiment of the present invention, the combustion process does not include any dedicated NO_(x) reduction process, i.e., for example, a selective catalytic NO_(x) reduction (SCR) or selective non-catalytic NO_(x) reduction (SNCR) process. Correspondingly, according to a preferred embodiment of the present invention, the combustion system does not comprise any dedicated means for NO_(x) reduction, i.e., for example, a NO_(x) catalyst or means for selective non-catalytic NO_(x) reduction. Reason for this is that any NO_(x) in the vent gas from the CPU is efficiently adsorbed in the adsorption unit, and the NO_(x) is desorbed and recycled back to the furnace. The recycled NO_(x) is efficiently reduced to N₂ in the furnace, which N₂ will then eventually be separated by the adsorption unit to the pass through gas stream which is released to the atmosphere, either directly or via the ASU. On the other hand, in order to avoid dissolving sulphur in the exhaust gas into the liquid CO₂ stream discharged from the CPU unit, the combustion system advantageously comprises an efficient SO_(x) capturing process, like a scrubber, in the system.

When the adsorption unit adsorbs all the CO₂ of the vent gas stream, it adsorbs in practice also some N₂ and O₂. At least when some zeolites are used for efficient adsorption of CO₂, a considerable amount of N₂ may also be adsorbed. In this case, the pass through gas stream may have a relatively high O₂ concentration, and it forms a high quality feed gas to the ASU. The pass through gas stream fed from the adsorption unit to the ASU consists, in addition to N₂ and O₂, also some Ar. The ASU removes the N₂ and separates O₂ to be conducted to the furnace. The ASU usually leaves a portion of the Ar in the product gas stream, and thus some accumulation of Ar in the circulating gas may result. In case Ar accumulation becomes a problem, it is possible to release at least a portion of the pass through gas to the atmosphere, or to modify the ASU to improve the Ar separation.

According to a most preferred embodiment of the present invention, the adsorbing material and/or operation conditions of the adsorption unit are selected in such a way that, simultaneously when CO₂ is efficiently adsorbed, also clearly more O₂ than N₂ is adsorbed in the adsorbing material. Suitable material may be, for example, activated carbon when used at suitable operating conditions. When using such an adsorption process, the pass through gas comprises mainly N₂ and only a relatively small amount of O₂. Thus, the pass through gas can especially in these conditions advantageously be released to the atmosphere instead of being used as a feed gas of the ASU.

In case the adsorption unit operates at a low temperature, the pass through gas is, before being released to the atmosphere, advantageously used as a coolant in the CPU and/or a cryogenic ASU. In some cases it is also advantageous to use at least a portion of the desorbed gas stream as a coolant of the inlet gas streams of one or more of the CPU and ASU, before it is conducted back to the furnace.

In case of using an adsorption process with efficient O₂ adsorption, the adsorbed O₂, i.e., all or at least most of the excess O₂ of the exhaust gas, is advantageously recycled back to the furnace, together with CO₂, as a part of the gas desorbed from the adsorption material. Due to the fact that all, or at least most, of the excess O₂ in the exhaust gas is recycled back to the furnace, the air stream introduced to the air separation unit can be limited to such providing, at least nearly, only a stoichiometric amount of oxygen for the combustion of the fuel. Thus, the power consumption of the ASU is mitigated.

The CPU comprises at least one condenser which very efficiently removes most of water from the exhaust gas and produces one or more streams of condensate. Simultaneously impurities, especially SO_(x), remaining in the exhaust gas, dissolve in the condensate and are thus also separated from the exhaust gas. According to a preferred embodiment of the present invention, the condensed water, together with impurities dissolved therein, is conducted as a makeup water into the scrubber arranged in the exhaust gas channel. Thereby, at least most of the recirculated impurities are captured in the scrubber. Because of the recirculation of the impurities, especially SO_(x), the scrubber does not need to be operated by high capturing efficiency.

In case the combustion system comprises a pulverized coal (PC) boiler, the scrubber is advantageously a desulphurizing wet scrubber, whereby the condensate may advantageously be added to the scrubbing solution, preferably alkaline scrubbing solution, of the scrubber. Thus, the pollutants in the exhaust gas, including SO_(x), HCl, HF and Pb, are discharged from the plant only together with the end product of the scrubber, typically dried solid gypsum. In case the combustion system comprises a circulating fluidized bed (CFB) boiler, most of the SO₂ generated in the combustion of the fuel is removed already in the furnace, by feeding an absorbent, such as limestone, into the furnace. Thus, the scrubber is only a supplementary sulphur removing device, advantageously a dry scrubber. Such a dry scrubber, which can in some cases also be used in a combustion system with a PC boiler, typically comprises an ash circulating cycle with an ash humidifier, and the condensate from the CPU is advantageously used as a moistening liquid in the humidifier. The dry scrubber comprises means, such as a bag filter, for collecting solid particles, i.e. ash and solid salts of the impurities from the exhaust gas. By feeding the condensate streams from the CPU to the scrubber, the processing and disposal of dirty condensates is eliminated and the need for adding water to the scrubber is minimized.

The recycling second exhaust gas stream and the gas stream desorbed from the adsorption unit can be introduced separately into the furnace, but according to a preferred embodiment of the present invention, the desorbed gas stream is mixed with the second exhaust gas stream, and the mixed gas stream is introduced into the furnace. Correspondingly, as is commonly known, the oxygen stream from the source of oxygen, such as ASU, can be introduced to the furnace as a separate gas stream or mixed with the recycling second exhaust gas stream. According to preferred embodiments of the present invention, the oxygen stream is introduced to the furnace mixed with the recycling second exhaust gas stream or the gas stream desorbed from the adsorption unit, or mixed with both the desorbed gas stream and the recycling second exhaust gas stream.

The present invention provides a simple and low-cost emissionless oxyfuel combustion process, in which the exit streams mainly consist of N₂ and Ar from the ASU, ash from the boiler, solid or liquid end products from the scrubber and high purity CO₂ from the CPU. Due to emission recycling, nearly complete CO₂ removal can be achieved.

When compared to conventional oxyfuel combustion process, the combustion process in accordance with the present invention provides savings from minimized excess O₂ production and potential relaxation of O₂ purity requirement. Due to the recycling of the gas desorbed from the adsorption unit, the gas flow to the CPU is slightly increased.

The above brief description, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the currently preferred, but nonetheless illustrative, embodiments of the present invention, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a combustion system comprising a CFB boiler, according to a first preferred embodiment of the present invention.

FIG. 2 is a schematic diagram of a combustion system comprising a PC boiler according to a second preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically discloses an oxyfuel combusting combustion system 10, comprising a circulating fluidized bed (CFB) boiler 12 with a furnace 14 for combusting carbonaceous fuel introduced to the furnace by fuel feeding means 16. The term furnace 14 is here used to refer to the actual furnace as well as to the conventional hot recycling loop and possible heat exchange chambers connected to the furnace. The combustion system 10 comprises an oxygen channel 20, for feeding oxygen from a source of oxygen 22, such as a cryogenic air separation unity (ASU), to the furnace. The ASU converts an incoming air stream 24 to a first stream 26 comprising mainly oxygen (O₂), and another stream, nitrogen (N₂) rich stream 28, which is released to the atmosphere or conducted to another application.

The combustion process produces exhaust gas, comprising a high amount, for example, about 75%, of carbon dioxide (CO₂) and having water and excess O₂ as its other main components. The exhaust gas is discharged from the furnace 14 via an exhaust gas channel 30 comprising conventional heat exchangers for producing steam, such as superheaters, reheaters and economizers, not shown in FIG. 1. In order to maintain temperature in the furnace 14 at a desired level, the fuel is advantageously combusted in the furnace by a combustion gas comprising O₂ and recycling exhaust gas, which recycling exhaust gas is conducted to the furnace along a recycling gas channel 32. The recycling gas channel may be connected with the oxygen channel 20 to form a combined combustion gas channel 34. It is also possible that a portion of the oxygen and recycling gas streams are combined before they are introduced to the furnace, but another portion of at least one of the oxygen and recycling gas streams is introduced to the furnace separately. It is also possible that multiple separate or combined gas streams, possibly with different compositions, are conducted to different locations of the furnace 14. Ash produced in the combustion of the fuel is removed from the furnace 14 by bottom ash removing means 36 arranged at the lower portion of the furnace.

In order to minimize the level of pollutants in the exhaust gas stream, the combustion system 10 may comprise conventional means for capturing pollutants. Especially the furnace 14 advantageously comprises means for feeding sulphur capturing agent 38, such as limestone, into the furnace. In order to avoid sulphur dissolving as an impurity in the liquid CO₂ stream produced in a CO₂ purifying and capturing unit (CPU) 40 arranged at an end portion of the exhaust gas channel 42, the exhaust gas channel comprises advantageously a dry scrubber 44 for minimizing the SO_(x) level of the exhaust gas.

Because the combustion gas does not comprise a significant level of N₂, the need for NO_(x) reduction is less than in air combustion. In order to capture NO_(x) originating from the fuel combustion, the furnace and/or upstream portion of the exhaust gas channel may comprise means for SNCR (selective non-catalytic NO_(x) reduction), i.e., means for feeding NO_(x) reagent 46, such as NH₃, at a suitable temperature into the exhaust gas. In some cases, the exhaust gas channel may comprise means for SCR (selective catalytic NO_(x) reduction), i.e. a NO_(x) catalyst and means for feeding a reagent, not shown in FIG. 1. However, for reasons to be explained later, advantageously the combustion system does not comprise means for either SNCR or SCR.

The portion of the exhaust gas which is not recycled to the furnace via the recycling gas channel 32, and which from now on is called a first portion of the exhaust gas, is conducted towards the CPU 40 via the end portion of the exhaust gas channel 42. The branch point of the exhaust gas stream portions may be located downstream of the scrubber 44, as in FIG. 1, or in another suitable position in the exhaust gas channel 30. The recycling gas channel 32 and/or the end portion of the exhaust gas channel 42 may advantageously comprise means for controlling the exhaust gas flow rate, for example, dampers, control valves or other flow restrictors, which, however, are not shown in FIG. 1.

The first portion of the exhaust gas flowing through the end portion of the exhaust gas channel 42 may contain a relatively high amount of water, typically, from about 10% to about 40%. Most of the water is advantageously removed upstream of or at the CPU 40 as one or more condensate streams 48. The water is to be removed prior to the CO₂ capture, in order to avoid harm that may otherwise be caused by the presence of frozen water (i.e., ice or ice particles) in the downstream stages. The drying step can advantageously be finalized to ppmv level by chemical de-moisture or adsorption.

The CPU 40 typically comprises several heat exchangers and compressors for cooling and compressing the first exhaust gas stream so as to produce the one or more condensate streams 48 and a stream of liquid CO₂ 52. A portion of impurities, especially SO₂, remaining in the first portion of the exhaust gas dissolve in the condensate streams 48 and is thus also separated from the exhaust gas. The one or more condensate streams 48 are advantageously recycled from the CPU 40 into the dry scrubber 44. A dry scrubber typically cornprises an ash circulating cycle with an ash humidifier, not shown in FIG. 1. The one or more condensate streams 48 are advantageously conducted to the scrubber 44 to be used as a moistening liquid in the humidifier. The scrubber 44 advantageously comprises a particle filter, such as bag filter, not shown in FIG. 1, for collecting and discharging solid particles 50, i.e. ash and solid salts of the impurities from the exhaust gas. By feeding the condensates from the CPU 40 to the scrubber 44, the need for adding water to the desulphurizing is minimized, and the need to process and dispose condensates of the CPU 40 is avoided.

The liquid CO₂ stream 52 produced in the CPU 40 preferably comprises at least about 90%, even more preferably at least about 95%, of the CO₂ existing in the first portion of the exhaust gas. The rest of the CO₂, as well as uncondensed gases, mainly O₂, N₂ and Ar, are discharged from the CPU 40 as a high pressure vent gas stream 54.

The vent gas stream 54 is conducted from the CPU 40 to an adsorption unit 56, comprising a bed of adsorbing material. The main function of the adsorption unit is to adsorb CO₂ remaining in the vent gas stream 54. The conditions in the CPU 40 are advantageously adjusted so that the vent gas stream 54 is close to CO₂ dew point, and the vent gas is suitable for very efficient CO₂ separation. The adsorption material and operating conditions of the adsorption unit 56 are advantageously selected so that almost all, preferably at least 90%, even more preferably at least 95%, of CO₂ remaining in the vent gas stream 54 is adsorbed in the adsorption material. The adsorbing material bed may advantageously comprise activated carbon, zeolites or other suitable materials. Because of the use of the adsorption unit 56, it is not necessary to aim for a very high CO₂ capturing efficiency in the CPU 40, and, for example, difficulties related to compressing to very high pressures and/or cooling to very low temperatures are avoided.

The adsorption unit 56 comprises conventional means for regenerating the adsorption material, not shown in FIG. 1. The regeneration is effected by changing the conditions in the adsorption unit so that at least a portion of the adsorbed molecules is desorbed from the adsorption material, whereby a desorbed gas stream 60 is formed. According to a preferred embodiment of the present invention, the adsorption unit 56 is a pressure swing adsorption (PSA) unit, which is regenerated by reducing the pressure to a suitable low pressure, typically about 1 bar. The adsorption unit 56 in practice usually comprises at least two parallel adsorption beds, not shown in FIG. 1, so that at least one bed can always be operational, even when regeneration of another bed is being made.

At least a portion, preferably all, of the desorbed gas stream 60 is advantageously recycled along a desorbed gas channel back to the furnace 14. According to the embodiment shown in FIG. 1, the desorbed gas stream 60 is combined with the second exhaust gas stream recycling to the furnace 14 along the recycling channel 32. The desorbed gas stream could alternatively be combined with, for example, the oxygen stream 26 being conducted along the oxygen channel 20 to the furnace 14, or it could be conducted directly to the furnace 14.

The portion of the vent gas stream 54 which is not adsorbed in the adsorption unit 56, so called pass through gas stream 58, comprises thus mainly O₂, N₂, and Ar. Thus, the pass through gas stream 58 is nearly CO₂ free. Assuming that all CO₂ of the vent gas stream 54 is adsorbed in the adsorption unit 56, not any CO₂ is released to the atmosphere, but, instead, all CO₂ remaining in the vent gas stream 54 is recycled back to the furnace 14. Thereby, the CO₂ content of the exhaust gas in the exhaust gas channel 30 increases to a level at which the amount of CO₂ captured in the CPU 40 equals to the amount of CO₂ produced in the combustion process.

When CO₂ of the vent gas stream 54 is adsorbed in the adsorption unit 56, all pollutants remaining in the vent gas stream, especially NO_(x), SO_(x) and CO molecules, as well as possible volatile organic compounds (VOC), are advantageously also adsorbed in the adsorbing material. Thus, when the desorbed gas stream 60 is conducted back to the furnace 14, all pollutants remaining in the vent gas stream 54 are also recycled back to the furnace. Thus, the combustion system renders possible an emissionless combustion of the fuel.

The combustion system forms two closed loops, direct recycling of the second portion of the exhaust gas along the recycling channel 32 and the recycling of the desorbed gas stream 60. Due to the above-described recycling of pollutants, conventional NO_(x) and SO_(x) reducing processes of the combustion system function very effectively, and it is, for example, possible to use relatively low Ca/S and NH₃/NO_(x) ratios in the SO_(x) and NO_(x) removing processes, respectively. In order to avoid dissolving of sulphur in the liquid CO₂ stream 52 discharged from the CPU unit 40, it is, however, advantageous to have an efficient desulphurizing device, advantageously a dry scrubber 44, arranged in the exhaust gas channel 32.

The source of oxygen 22 is advantageously a cryogenic air separation unit (ASU). When adsorbing CO₂ in the adsorption unit 56, typically also other components of the vent gas stream 54 are adsorbed. At least when using some zeolites as adsorption material, a considerable amount of N₂ may be adsorbed, whereas most of the O₂ does not adsorb. Thus, in these conditions, the adsorption unit 56 produces a pass through gas stream 58 which may have a relatively high O₂ concentration, and which can thus advantageously be used as a high quality feed gas for the ASU. Correspondingly, FIG. 1 shows an embodiment of the present invention, wherein the pass through gas stream 58 of the adsorption unit 56 is conducted along a pass though gas channel as a feed gas to an ASU, which is especially advantageous in case the average O₂ content of the pass through gas is higher than that of air. Before the pass through gas stream 58 is conducted to the ASU, it is advantageously expanded to a suitable pressure in an expander, not shown in FIG. 1.

Most of the pollutant molecules, which are recycled back to the furnace 14 together with the desorbing gas stream, are advantageously destructed in the furnace, and so the pollutant levels in the exhaust gas do not rise to a very high level. Especially the recycled NO_(x) molecules are efficiently reduced to N₂. Thus, the present inventors have surprisingly observed that the combustion system 10 may advantageously be without means for either SNCR or SCR. Then, any NO_(x) formed in the combustion process is recycled from the adsorption unit 56 back to the furnace where it is reduced to N₂, which will eventually be vented to the atmosphere, either directly or via the air separation unit.

The recycling second exhaust gas stream and the desorbed gas stream can be introduced separately into the furnace 14, but FIG. 1 shows a preferred embodiment of the present invention, wherein the desorbed gas stream is conducted to the recycling gas channel 32 so as to combine the desorbed gas stream and recycling second exhaust gas stream to be conducted as a combined gas stream into the furnace 14. According to the embodiment shown in FIG. 1, the oxygen stream from the source of oxygen 22 is also combined with the combined gas stream of desorbed gas and recycling second exhaust gas stream.

FIG. 2 schematically discloses another oxyfuel combusting combustion system 10′. Generally identical or similar elements in FIGS. 1 and 2 are identified with same reference numbers. The combustion system 10′ differs from that shown in FIG. 1 in that it comprises a pulverized coal (PC) boiler 12′, and the furnace 14 does not comprise means for feeding a sulphur binding agent into the furnace. Moreover, the exhaust gas channel 30 advantageously comprises, in order to effectively capture SO₂ from the exhaust gas, a conventional wet scrubber 44′, wherein SO₂ is captured from the exhaust gas to a sulphur absorbent containing washing liquid. The condensate streams 48 discharged from a CPU unit 40 are advantageously conducted to the scrubber 44′ to be combined with the washing liquid of the scrubber. SO₂ and possible other pollutants are advantageously removed from the scrubber 44′ as a dry product 50′, such as gypsum.

When using the embodiment shown in FIG. 2, the adsorbing material and/or operation conditions of the adsorbing unit 52 are advantageously selected so that, when CO₂ of the vent gas stream 54 is adsorbed in the adsorption material, also most of the O₂ in the vent gas stream 54, at least clearly more O₂ than N₂, is adsorbed. Suitable adsorption material may be, for example, activated carbon when used at suitable operating conditions. The pass through gas stream 58 then mainly comprises N₂ and Ar, and only a relatively small amount of O₂. The oxygen content of the pass through gas stream 58 is advantageously less than about 21%, i.e. less than that of the ambient air. Therefore, the pass through gas stream 58 is advantageously released to the atmosphere, instead of being conducted to a feed gas inlet of the ASU. The adsorbing unit 56 advantageously operates at a low temperature, and the pass through gas channel is connected to the CPU 40 in order to use the pass through gas stream 58 as a coolant of the first portion of the exhaust gas. According to an alternative embodiment of the present invention, not shown in the FIGs, the pass through gas stream 58 is conducted to a cryogenic ASU, to be used as a coolant for the ASU.

In order to completely combust the fuel fed into the furnace, it is in practice necessary to introduce to the furnace more than a stoichiometric amount of oxygen. The oxygen which is not used for combustion remains then in the exhaust gas as an excess oxygen, amounting typically to about 3% of the exhaust gas. According to a preferred embodiment of the present invention, the adsorption material and/or adsorption conditions in the adsorbing unit 56 are selected so that most of the excess O₂ of the exhaust gas, which forms usually a considerable portion of the vent gas stream 54, is adsorbed in the adsorption unit 56. Thus, when the desorbed gas stream 60 of the adsorption unit is recycled back to the furnace 14, most of the excess O₂ of the exhaust gas, is also recycled back to the furnace. Because of this recycling of the excess O₂, the need for fresh oxygen from the source of oxygen 22 is minimized. In practice, the air stream introduced to an ASU can then be limited to such providing only about stoichiometric amount of oxygen for the combustion of the fuel, and the power consumption of the ASU is mitigated.

While the invention has been described herein by way of examples in connection with what are at present considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features and several other applications included within the scope of the invention as defined in the appended claims. 

1. A method of combusting carbonaceous fuel in a combustion system, which combustion system includes a source of oxygen and a furnace, the method comprising the steps of: (a) feeding fuel and combustion gas comprising oxygen and recycling gas into the furnace for combusting the fuel with the oxygen and producing exhaust gas comprising CO₂, water and excess oxygen as its main components; (b) conducting the exhaust gas discharged from the furnace into a scrubber so as to remove pollutants from the exhaust gas; (c) dividing the exhaust gas into a first exhaust gas stream and a second exhaust gas stream and conducting the second exhaust gas stream as a recycling gas stream into the furnace; (d) conducting the first exhaust gas stream into a CO₂ purification and capturing unit (CPU) to produce one or more condensate streams, a purified liquid CO₂ stream and a vent gas stream comprising remaining CO₂; (e) discharging the purified liquid CO₂ stream from the combustion system; (f) conducting the vent gas stream into an adsorption unit so as to adsorb compounds, including remaining CO₂, from the vent gas stream to an adsorbing material and to produce a pass through gas stream; and (g) regenerating the adsorbing material to produce a desorbed gas stream, comprising at least a portion of the adsorbed compounds, and conducting at least a portion of the desorbed gas stream into the furnace.
 2. The method according to claim 1, wherein the vent gas stream discharged from the CO₂ purification and capturing unit is at a pressure from about 20 bar to about 40 bar and at a temperature from about −50° C. to about 20° C.
 3. The method according to claim 1, wherein the adsorption unit is a pressure swing adsorption unit.
 4. The method according to claim 1, further comprising a step of conducting at least a portion the one or more condensate streams into the scrubber.
 5. The method according to claim 1, wherein the source of oxygen is an air separation unit and at least a portion of the pass through gas stream is conducted as a feed gas to the air separation unit.
 6. The method according to claim 1, wherein the adsorption unit adsorbs more efficiently O₂ than N₂, and at least a portion of the pass through gas stream is conducted to the atmosphere.
 7. The method according to claim 1, wherein at least a portion of one or more of the pass through gas stream and the desorbed gas stream is conducted as a coolant to one or more of the CPU and the source of oxygen.
 8. The method according to claim 1, further comprising the step of mixing the desorbed gas stream with the recycling gas stream prior to their introduction into the furnace.
 9. The method according to claim 1, further comprising the step of mixing a stream of oxygen from the source of oxygen with one or more of the desorbed gas stream and the recycling gas stream prior to their introduction into the furnace.
 10. A combustion system for combusting carbonaceous fuel, the combusting system comprising: a source of oxygen; a furnace; means for feeding fuel and a combustion gas comprising oxygen and recycling gas into the furnace for combusting the fuel with the oxygen and producing exhaust gas comprising CO₂, water and excess oxygen as its main components; an exhaust gas channel connected to the furnace for discharging the exhaust gas from the furnace; a scrubber arranged in the exhaust gas channel for removing pollutants from the exhaust gas; means for dividing the exhaust gas into a first exhaust gas stream and a second exhaust gas stream; a recycling channel for conducting the second exhaust gas stream as a recycling gas stream into the furnace; an end portion of the exhaust gas channel for conducting the first exhaust gas stream into a CO₂ purification and capturing unit (CPU) to produce one or more a condensate streams, a purified liquid CO₂ stream and a vent gas stream comprising remaining CO₂; means for discharging the purified liquid CO₂ stream from the combustion system; an adsorption unit arranged to adsorb compounds, including remaining CO₂, from the vent gas stream to an adsorbing material and to produce a pass through gas stream; means for regenerating the adsorbing material to produce a desorbed gas stream, comprising at least a portion of the adsorbed compounds; and a desorbed gas channel for conducting at least a portion of the desorbed gas stream into the furnace.
 11. The combustion system according to claim 10, wherein the adsorption unit is a pressure swing adsorption unit.
 12. The combustion system according to claim 10, further comprising means for conducting at least a portion of the one or more condensate streams into the scrubber.
 13. The combustion system according to claim 10, wherein the source of oxygen is an air separation unit and the combustion system comprises means for conducting at least a portion of the pass through gas stream as a feed gas to the air separation unit.
 14. The combustion system according to claim 10, wherein the adsorption unit comprises adsorption material suitable for adsorbing O₂ more efficiently than N₂, and at least a portion of the pass through gas stream is conducted to the atmosphere.
 15. The combustion system according to claim 10, further comprising means for conducting at least a portion of one or more of the pass through gas stream and the desorbed gas stream as a coolant to one or more of the CPU and the source of oxygen.
 16. The combustion system according to claim 10, further comprising means for mixing the desorbed gas stream with the recycling gas stream prior to their introduction into the furnace.
 17. The combustion system according to claim 10, further comprising means for mixing a stream of oxygen from the source of oxygen with one or more of the desorbed gas stream and the recycling gas stream prior to their introduction into the furnace. 